Inductive heating coil shielding



Auge Ti, 1945. 1.. w. GREGORY Erm. 2,381,278

INDUCTIVE HEATING COIL SHIELDING Filed Nov. 5, 1942 HNVENTORS uf/2er A/ Greyar y ATTOR NEY Patented Aug. 7, 1.945VY vINDUC'IIVE HEATING COIL SHIELDING Luther W. Gregory, Baltimore, and Stanley S.

Schneider, Halcthorpe, Md., assignors to Westinghouse Electric Corporation,

Em Pittsburgh,

Pa., a corporation of Pennsylvania Application November 5, i942, Serial No. 464,672

12 Claims.

' Our invention relates to high-frequency inductive heating-coils 'for operating at such a large power-input that the danger of corona, or other space-current discharges, is an important consideration in the design, by reason of the high electrostatic stress between the high-voltage portions of the coil and the work being heated. More particularly, our invention relates to a coil-construction including improved means for shielding the workagainst space-current discharges, while offering but slight hindrance to the axial magnetic lines of force within the work being heated.

Although our invention, in its broadest aspects, is not limited to any .particularfield of 4applica-- tion, the most important present ileld of application for our invention is in connection with the tinplate-iiowing apparatus which is described and claimed in an application oi' Glenn E. Stoltz and Robert M. Baker, Serial No. 464,040, illed October 31, 1942,and assigned to the Westinghouse Elec-A tric 8L Manufacturing Company. In this tinplateowing application o1' our invention, a miles-long strip 'of steel, or other magnetizable metal, is caused to run. at a very high rate o f speed, through a tinning "line or mill, in which various operations are successively performed on the both sides, quickly heating it to a temperature of 452 F., or slightly thereover, for the purpose oi' melting the tlnplate and causing it to iiow evenly and glossily over the surface of the steel to enhance its corrosion-resistant properties, as well as improving its appearance, subsequently quenchlng the strip, and performing various additional operations thereon, having nothing to do with the present invention, except that it is necessary togauge the rate of heat-input into thestrip to strip, including electrolytically tinplatlng it on correspond to the large poundage of material which has to be heated per minute, preferably within as short a strip-length as is practically possible.

Themest satisfactory method of heating such a fast-moving strip, and practically the only method of controllably introducing the necessary quantity of heat within an acceptably short length o1' the fast-moving strip, is the inductive heating method, in which the strip is passed axially through an inductive heating-coll which is supplied -with a large amount of pOwer from a highheating-coil, the coil-voltage becomes so high as 4to be one of the limiting or controlling features of the coil-design, and the danger of corona to the strip, or other work being heated, is an important phase of the high-voltage limitation.

The object of our present invention is to provide grounded electrostatic held-shielding means which is associated with the inductive heating- Y coil in a novel manner whereby various improved results `are obtained, over and above previous, 4known methods of induction-coil shielding,

Two exemplary forms of embodiment of our invention are illustrated in the accompanying drawing, wherein l Figure 1 is a diagrammatic view of a tinplateflowing application of our invention, including a longitudinal cross-sectional view through the induction-coil structure, looking edgewise on the strip being heated,

Fig. v2 is a transverse cross-sectional view illustrating the relation between the con, the sinn.

the linersurrounding the strip within the coil. the airgap between the liner and the coil, and the grounded shielding coating painted on the liner,

Fig. 3 is a cross-sectional view of another form of embodiment of our invention, which is illustrated in connection with a wire-heating apparatus, y

Fig. 4 is an elevational view oi a portion of the liner on a reduced scale, showing a modiiled form of shielding-means, and

Fig. -5 is a vertical sectional view of the modiilcation shown inFig. 4.

In Figures 1 and 2, we show our invention as being applied to the rapid inductive heating of a rapidly moving tinplated steel strip 4, which generally has a gauge of between 0.008 and 0.011 inch, with a width of the order of 28 to 36 inches, more or less, and which is moving, in the direction of its length, at a speed which may be of the order of a thousand feet per minute, more or less. This strip 4 passes axially downwardly through an inductive heating coil l, comprising a'plurality oi" turns of a hollow conductor, wound in a single layer, and cooled by circulatingl water or other coolant, which may enter the coil at'a grounded midpoint 0, and which may leave the coil at two insulated terminals 1 and l. The coil-terminals are electrically connected, at l and I0, to a high- ,power oscillation-generator I2, which may have 'mechanical strength for the particular applica.-

tion of the invention. The liner Il has an opening Il therethrough, through which the strip or elongated member l is threaded, and the entrance-end o f the opening Il may be provided with a rubber guiding-means Il for reducing the probability of contact between the strip l and the inner surface of the liner I2.

The outer periphery of the liner Il is preferably spaced. by an airgap I1, from the inner periphery of the coil l, particularly when the work l to be heated is of 4magnetizable material, so that the heating-eilicieney is less deleteriously aifected'by enlarging the coil l sufiiciently to provide the airgap-space I1, or a space which is occupied by substantially nothing but a gaseous ambient medium, particularly in the portions of the space between the ungrounded or high-voltage portions or terminals of the coil and the work l being heated. This airgap I1 is advantageous, because air hasa specific inductive capacity, or dielectric constant, of substantially unity, which is much lower than the dielectric Iconstant ci any available solid insulating material, so that moet of the electrostatic voltage-drop, between the high-voltage ends of thecoil and the work 4 being heated, which is usually grounded, takes place in the airgap 1, rather than in the solid insulation of the liner Il. The problem of corona is experienced only. when the strip l swings out of its central position of alignment,

and comes intol contact, or near-contact, with the inner surface of the liner Il, and under these conditions, the airgap I1 between the liner and the coil is' an elective means for reducing the electrostatic field-stress between the liner and `the strip I or other work'being heated.

In accordance with our present invention. we provide a means whereby the voltage of the `coil can be pushed higher than can be provided for. in any practical design, by means ci the airgap II alone. For the extremely high-power applications for which. our invention is intended. we provide, therefore, a suitable grounded electrostatic eld-shielding means,which is associated with the liner Il, for preventing corona between the strip 4 and the liner i2. Preferably this corona-preventing means is in the form oi a grounded semi-conducting adherent coating Il, which is painted on a surface of the liner i3, preferably on the outer surface thereof, as shown in Figs. i and 2, this coating being suitably grounded at some point, as indicated at Il in Fig. 1, wherein the grounding point is 'at an extended end of the nner n; which extends muy beyond the length of the coil l, into a region where the alternating magnetic field is relatively weak.

The grounded semi-conducting adherent coating Il may be a coating of very finely divided graphite. Suspended lin water. and .known ns aquadag," or it may be any' other semi-conducting adherent coating, such 'as a compound of varnish, or other'insulating binder, with a semiconducting powdered material, such as' woodassure charcoal or powdered titanium dioxide which has been partially reduced in a hydrogen furnace. The painted shielding-coating Ii necessarily has such a high resistivity or such a thinness, or both, that it is not materialLv heated by the alternating magnetic flux of the coil, which is another way of saying that it oifers but slight hindrance to the axial magnetic lines of force which reach the strip l or other work being heated. The thickness of the semi-conducting coating Il is necessarily greatly exaggerated, in Figs, 1 and 2, in order that it may be shown at all.

In operation, our grounded semi-conducting shield Il holds down the voltage of the insulating liner I2 to a value which is sufficiently close to ground, at all points, so that no corona is experienced between the work I and the liner I2. By this means, it is possible to extend the powerinput into the coil l to any desired voltage which is free of any consideration of corona or other space-current discharge playing upon the work I. Of course, when this is done, the internal cross-sectional area embraced by the coil 5 must be enlarged. s0 that the airgap Il will be of sumcient thickness to withstand arcing from the inner periphery of the coil I to the grounded shield I8, and sometimes it is necessary, also, to avoid excessive corona-discharges between the coil and the shield. It has been found, however, particularly wherethe work l is magnetizable, as is the case in tinplate-ilowing apparatl that the eiliciency of heating is not very greatly reduced by enlargement of the inductive heating-coil l, so that it is possible to rather extensively increase the radial length or thickness of the airgap I1 without very serious reduction in emciency, when the paramount consideration is the introduction of as much heat as possible into as short a length of the strip l as possible.

The insulating liner l'and the coil i must obviously both be suitably supported, in a manner which does not interferewlth the'airgap I1 between them. In Fig. l, we have shown a form of support which is described and claimed in an application oi Harold J. Prostick and Elvyn ll. Sollie, Serial No; 464,042, led October 3'1, i942, assigned to the Westinghouse Electric la Manufacturing Company. The outer periphery o! the coil 5 is embedded in a molded insulating material 2l, which may be asphalt, or a casting resin, or other suitable insulating material.,

Both the coil-support 2| and the liner Il extend axially beyond theends of the coil l, and the space between them is closed, at each end, by

' an annular insulating end-closure 2 2, which preferably makes a dust-tight compartment for completely enclosing the coil I.

In Fig. 3, we show a form of embodiment oi' our invention in which we utilise two concentric insulating liners, the inner liner I3 being sur- `ro|mded by an outer liner 2l of solid insulating material, which may either i'it rather snugly within the coil l', or the coil may actually be wound upon the outer liner 22 as a mandrel. The space between the two liners I2' and 2l is il'lled with a quantity of flowed` material Il', which includes a substantially insulating binder, and a semi-conducting powdered material or other means which constitutes a semi-conducting shielding-means for controlling the electrostatic field, the shielding-means being grounded as indicatedat I9'. In this manner, Substantially the entire electrostatic stress is taken up by the outer liner 23, between the outer and inner peripheries thereof, so that there is practically no,A or only moving wire 26, which may or may not be of al magnetizable metal.

While we have had good success heretofore with a semi-conducting shielding-coating I8 extending all the way around the periphery of the insulating liner I3-I4` there may arise cases, particularly at the higher radio-frequencies or at high magnetizing forces, where the circumferential eddy-current Voltage may be too high for satisfactory results with this type of circumferentially extending shield, making a complete eddy-current path around the periphery of the liner I3.

Accordingly, in Figs. 4 and 5, we have shown a modified form of shielding-coating 25 which is provided with axially extending unpainted-strips 25', except at the end, which is attached, at 25", to the grounded connection I9. Since the midpoint of the inductive heating coil 5 is grounded, no shielding is needed opposite this point, and therefore two painted-on shields may be utilized. one grounded at I9 and the other at I9', the grounding connections being at points beyond the respective ends of the coil 5, where the magnetic iiux-density is weak, so that the eddy-current interruptions 25' are not needed.

In order to avoid corona across the axially extending bare-places 25 between the strips of the shield 25, the whole surface may be painted over with a semi-conducting coating 26, Figs. 4 and 5, which extends around the entire circumference, but has a sufficiently high resistivity so that eddycurrent heating is not excessive. In this manner, the composite shield 25-26 has a relatively low axial resistivity, along the strips of the iirst coating 25, and a much higher circumferential resistivity, which is necessary or desirable in order to reduce the eddy-current heating of the shield. The two coatings 25 and 26 may be of the same or different coating-materials, and of the same or different thicknesses, so as to have `the same or different linear resistivities.

While we have illustrated our invention in two preferable or illustrative forms of embodiment, we wish it to be understood that our invention, in its broadest aspects, is not limited to these precise forms, and we desire that the appended claims shall be accorded the broadest construction consistent with their language and the prior art.

We claim as our invention:

1. Apparatus adapted to heat an elongated work tn be heated within a gaseous medium at approximatelv atmospheric pressure, said apparatus including. in combination, an induction heating coil comprising a plurality of turns of a cooled conductor wound in a single layer, means for supplyincr such a large high-frequency current to the coil that the danger of space-current discharges from the coil to the work due to a high electrostatic stress is an important consideration in the design. a solid insulating separating-means disposed within the coil with portions of said separating-means adapted to be on opposite sides of the work to be heated, means for supporting the separating-means and the coil in such manner that most of the space between the separatingmeans and the high-voltage portions of the coil is substantially nothing but a gaseous ambient medium, said gaseous space being adequate to take most of the voltage-drop between the highvoltage portions of the coil and the work being heated, and grounded electrostatic field-shielding means in intimate contact with the separatingmeans for guarding the work being heated against space-current discharges while offering but slight hindrance t0 the axial magnetic lines of force within the work being heated.

2. Apparatus adapted to heat an elongated work to be heated within a gaseous medium at approximately atmospheric pressure, said apparatus including, in combination, an induction heating coil comprising a plurality of turns of a cooled conductor wound in a single layer, means for supplying such a large high-frequency current to the coil that the danger of space-current discharges from the coil to the work due to a high electrostatic stress is an important consideration in the design, a solid insulating separating-means disposed within the coil with portions of said separating-means adapted to be on opposite sides of the work to be heated, means for supporting the separating-means and the coil in such manner that most of the space between the separatingmeans and the high-voltage portions of the coil is substantially nothing but a gaseous ambient medium, said gaseous space being adequate to take most of the voltage-drop between the highvoltage portions of the coil and the work being heated, and a grounded semi-conducting adherent coating on a surface of the separating-means.

3. Apparatus adapted to heat an elongated work to be heated within a gaseous medium at approximately atmospheric pressure, said apparatus including, in combination, an induction heating coil comprising a plurality of turns of a cooled conductor wound in a single layer, means for supplying such a large high-frequency current to the coil that the danger of space-current discharges from the coil to the work due to a high electrostatic stress is an important consideration in the design, a solid insulating liner disposed within the coil and having an axially extending opening therein for accommodating the work to be heated, the outer periphery of the liner being spaced from the inner periphery of the coil and being separated from at least the high-voltage portions of the coil essentially by a gaseous ambient medium, and grounded electrostatic held-shielding means in intimate contact with the liner for guarding the work being heated against space-current discharges while offering but slight hindrance to the axial magnetic lines of force within the work being heated.

4. Apparatus adapted to heat an elongated work to be heated within a gaseous medium at approximately atmospheric pressure, said apparatus including, in combination, an induction heating coil comprising a plurality of turns of a cooledv conductor wound in a single layer, means for supplying such a large high-frequency current to the coil that the danger of space-current discharges from the coil to the work due to a high electrostatic stress is an important consideration in the design. a solid insulating liner disposed within the coil and having an axially extending opening therein for accommodating the work to be heated, the outer periphery of the liner being spaced from the inner periphery of the coil and being separated from at least the high-voltage portions of the coil essentially by a gaseous ambient medium. and a. grounded semi-conducting adherent coating on a surface of the liner for guarding the work being heated against space-current discharges while offering but slight hindrance to the axial magnetic lines of force within the work being heated.

5. Inductive heating apparatus for heat-treat- -ing a continuously moving elongated member of magnetizabie metal moving in the direction of its length, said apparatus comprising an induction heating coil comprising a plurality of turns of a cooled conductor wound in a single layer, means for supplying such a large high-frequency current to the coil that the danger of space-current discharges from the coil t the elongated member due to a high electrostatic stress is an important consideration in the design, a solid yinsulating separating-means disposed within the coil with portions of said separating-means adapted to be on opposite sides of the elongated member, means for supporting the separating-means and the coil in such manner that most of the space between the separating-means and the high-voltage portions of the coil is substantially nothing but a gaseous ambient medium, said gaseous space being adequate to take most oi' the voltage-drop between the high-voltage portions of the coil and the elongated member, and grounded electrostatic fleld-shielding means in intimate contact with the separating-means for guarding the elongated member against space-current discharges while oiiering but slight hindrance to the axial magnetic lines of force within the elongated member.

6. Inductive heating apparatus for heat-treating a continuously moving elongated member of magnetizable meta1 moving in the direction of its length, said apparatus comprising an induction heating coil comprising a plurality oi' turns of a cooled conductor wound in a single layer, means for supplying such a large high-frequency current to the coil that the danger oi.' space-current discharges from the coil to the elongated member due to a high electrostatic stress is an important consideration in the design, a solid insulating separating-means disposed within the coil with portions of said separating-means adapted to be on opposite sides of the elongated member, means for supporting the separating-means and the coil in such manner that most of the space between the separating-means and the high-voltage portions of the coil is substantially nothing but a gaseous ambient medium, said gaseous space being adequate to take most of the voltage-drop between the high-voltage portions of the coil and the elongated member, and a grounded semiconducting adherent coating on a surface of the separating-means.

7. Inductive heating apparatus for heat-treating a continuously moving elongated member of magnetizable meta1 moving in the direction of its length, said apparatuscomprising an induction heating coil comprising a plurality of turns of a cooled conductor wound in a single layer, means for supplying such a large high-frequency current to the coil that the danger of space-current discharges from the coil to the elongated member due to a high electrostatic stress is an important consideration in the design, a solid insulating liner disposed within the coil and having an axially extending opening therein i'or accommodating the elongated member, the outer periphery of the liner being spaced from the inner periphery of the coil and being separated from at least the highvoltage portions of the coil essentially by a gaseous ambient medium, and grounded electrostatic held-shielding means in intimate contact with the liner for guarding the elongated member against space-current discharges while oilering but slight hindrance to the axial magnetic lines of force within the elongated member.

8. Inductive heating apparatus for heat-treating a continuously moving elongated member of magnetizable meta1 moving in the direction of its length, said apparatus comprising an induction heating coil comprising a plurality of turns oi a cooled conductor wound in a single layer, means for supplying such a large high-frequency cur- 'rent to the coil that the danger of space-current discharges from the coil to the elongated member due to a high electrostatic stress is an important consideration in the design, a solid insulating liner disposed within the 4coil and having an axially extending opening therein for accommodating the elongated member, the outer periphery of the liner being spaced from the inner periphery of the coil and being separated from at least the highvoltage portions of the coil essentially by a gaseous ambient medium, and a grounded semi-conducting adherent coating on a surface of the liner.

9. Inductive heating apparatus for heat-treating a continuously moving elongated member of conducting material moving in the direction of its length, said apparatus comprising an induction heating coil comprising a plurality of turns of a cooled conductor wound in a single layer, means for supplying such a large high-frequency current to the coil that the danger of space-current discharges from the coil to the elongated member due to a high electrostatic stress is an important consideration in the design, a. solid insulating liner disposed within the coil and having an axially extending opening therein for accommodating the elongated member, means for supporting said liner and said coil and means for providing therebetween a dielectric having a dielectric strength adequate to withstand substantially the entire voltage-drop between the coil and the elongated member, a quantity of flowed material in intimate contact with the outer periphery of said liner, between said liner and said dielectric, said flowed material including a substantially insulating binder and means for constituting a shieldingmeans for controlling the electrostatic field, and means for grounding the shielding-means without materially hindering the magnetic field, and means for grounding the shielding-means.

10. The invention as deiined in claim 9, characterized by said shielding-means comprising a .grounded semi-conducting adherent coating on a surface of the liner, said coating having a plurality of axially extending strips of coating, separated by unpainted strips, except at the grounded end of the coating, said grounded end of the coating being at a point axially beyond the end of the induction heating coil and hence partially out of its magnetic field.

11; The invention as defined in claim 9, characterized by said shielding-means comprising a grounded semi-conducting adherent coating on a surface of the liner, said coating having a plurality of axially lextending strips of coating, separated by unpainted strips, except at the grounded end of the coating, said grounded end of the coating being at a point axially beyond the end of the induction heating coil and hence partially out of its magnetic iieid, and another semi-conducting acterized by said shielding-means having a rela.- tively iow axial resistivity and a.' much higher circumferential resistivity, being continuous circumferentially.

LUTHER W. GREGORY. STANLEY S. SCHNEIDER.. 

